DFT method has been applied to calculate pK(a) and g-tensors values of semiquinone radicals mimicking the radicals present in humic acids. The results obtained support the hypothesis that naturally occurring stable semiquinone radicals (native form) are trapped in the humic acid macromolecule matrices which unables their direct involvement in the acid-base equilibrium with short-lived radicals (transient form). The differences between calculated g parameters for protonated and deprotonated model radicals are similar to those observed for humic acids allowing to attribute the stable radicals to protonated semiquinones and the short-lived radicals to deprotonated forms. (C) 2008 Elsevier B.V. All rights reserved.